Damage Evolution of a Type IV Composite Pressure Vessel Based on High-Frequency Acoustic Emission Signals and Background-Energy Analysis

This study investigates damage evolution in a polymer-lined, fully wrapped Type IV composite pressure vessel containing artificial local defects under stepwise hydrostatic loading. A 70 MPa vessel was pressurized in 20 MPa increments to 160 MPa, while acoustic emission (AE) was monitored continuously. Ball-impact calibration established the relationship between input mechanical energy and AE waveform energy. Three high-frequency criteria based on waveform and band-limited energy ratios were used to identify fiber-fracture-related high-frequency signals, while ratios between adjacent background-energy characteristic points were used to identify background-energy oscillation (BEO). The calibration yielded a mechanical-to-AE energy conversion coefficient of 1.92354 × 10−6 with R2 = 0.9707. No high-frequency signals or BEOs were detected during the 20–80 MPa holds. At 100 MPa, two high-frequency signals and four BEOs occurred, with a maximum adjacent-point ratio of 4.63, suggesting the coexistence of high-frequency responses consistent with fiber-dominated damage and continuous weak AE activity. At 140 MPa, three high-frequency signals indicated the re-emergence of localized high-frequency activity after stress re-equilibration. At 160 MPa, 23 high-frequency signals, a maximum C1 of 1330.434, and six BEOs reflected pronounced damage-related AE activity in the composite overwrap. The combined approach captures staged damage evolution and supports AE-based integrity assessment of composite pressure vessels.